1. Field of the Invention
In the art, there is a need for a method for measuring the particle size of ultra-small particles which are suspended in a fluid. More particularly, this concerns particles whose size is typically in the range of 1-3000 nm, but the particles can also be smaller than 1 nm.
The particles can be liquid or solid.
Examples of fields of application where the above-mentioned need exists, are:
environmental technology: measurement of aerosols, for instance soot particles in air, asbestos particles in air. PA1 biology: measurement of, for instance, virus particles in air, pollen in air. PA1 production-technology: measurement of, for instance, dust particles in air in so-called "clean rooms"; production of ultra-fine particles in a gas or a liquid (for instance dyes or medicines). PA1 medical analysis: measurement of body fluids, for instance blood composition, and the measurement of deposition of particles in the human body, in particular in the lungs.
2. Description of the Related Art
The need mentioned has existed for some time already, and measuring methods have already been developed to enable such measurements as mentioned to be carried out. An example of such a measuring method, known per se, is photon correlation spectroscopy, hereinafter designated as PCS. For an extensive description of this measurement technique, reference is made to the professional literature, such as, for instance, the article "Measurement of Aerosols in a Silicon Nitride Flame by Optical Fiber Photon Correlation Spectroscopy" by M. A. van Drunen et al in J. Aerosol Sci, 1994, vol. 25, no. 5, pp. 895-908. More particularly, in Chapter 2 of that article the theory underlying PCS is set out.
As explained in that article, PCS is based on the fact that particles suspended in a fluid undergo a Brownian movement, with the movement frequency of the particles being dependent (inter alia) on their size: the smaller the particles, the greater that frequency. A measuring signal representing that movement frequency can be derived from light which is reflected by the particles, more particularly from the fluctuations in the intensity of that light, which fluctuations, moved over a particular delay time, are correlated with themselves.
During the performance of a measurement, the measuring apparatus only "sees" a relatively small measuring volume, which is to say that only light signals from the particles present in that measuring volume are processed. In practice, that measuring volume typically has a magnitude of the order of 10.sup.-6 cm.sup.3. The strength of the measuring signal, that is, the intensity of the light received from that measuring volume is dependent, inter alia, on the concentration of the particles and more particularly on the number of particles present in the measuring volume: the more particles are present in the measuring volume, the more particles contribute to the measuring signal, that is, the greater that intensity.
A problem presenting itself here is based on the fact that the particles have a kinetic energy, that is, a velocity dependent on the temperature, as a result of which some particles will leave the measuring volume while other particles will enter the measuring volume. As a consequence, the number of particles actually present in the measuring volume at a particular time will not be constant but fluctuate over time. This fluctuation in the number of particles causes a second intensity fluctuation in the measuring signal, which influences the measuring signal. This effect is negligible in the case of relatively large numbers of particles because then the fluctuation in the number of particles is negligible with respect to the total number of particles. At low concentrations, however, in particular when the number of particles in the measuring volume is less than about 200, a noticeable effect occurs, which is greater when the number of particles in the measuring volume is smaller. The influence on the measuring result is such that the measured size of the particles differs from the actual size; more particularly, the measured size is greater than the actual size. In consequence, it has been assumed heretofore that PCS is only useful in cases of sufficiently high particle concentrations, as has been noted in Chapter 1 of the above-mentioned article, with reference to the article "Analysis of a Flowing Aerosol by Correlation Spectroscopy: Concentration, Aperture, Velocity and Particle Size Effects" by R. Weber et al in J. Aerosol Sci., 1993, vol. 24, p.485.